1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor with a shield device for solving carrier cross-talk problems.
2. Description of the Prior Art
The image sensor such as complementary metal oxide semiconductor (CMOS) or charge coupled device (CCD) is a silicon semiconductor device designed to capture photons and convert them into electrons. Electrons, once converted, must then be transferred and converted again to voltage which can be measured and turned into digital data. The photoconductor-on-active-pixel (POAP) image sensor has been studied to pursue advantages over the conventional CCD or CMOS image sensor. The POAP image sensor has a hydrogenated amorphous silicon (α-Si:H) based structure stacking on CCD or CMOS elements. The high fill factor brought by its stacking structure will provide the full of pixel area to be available for photo sensing, thereby achieving the high quantum efficiency in conjunction with the direct energy transition of α-Si:H material. However, this type of sensor still has cross-talk, image lag, and dark leakage signal problems in the past study. In particular, the problem of carrier cross-talk across adjacent pixels causes the serious resolution and uniformity degradation at the photo response, also brings the color cross-talk over the pixels that produces the poor color fidelity.
Referring to FIGS. 1-2, FIG. 1 is a cross-sectional schematic diagram of a POAP image sensor 10 according to the prior art, and FIG. 2 is a potential simulation diagram of adjacent pixel electrodes shown in FIG. 1. The prior-art image sensor 10 comprises a plurality of pixels 14a, 14b, a dielectric layer 16 disposed on a substrate 12, a plurality of pixel circuit (not shown) disposed in each pixel 14a, 14b, a plurality of pixel electrodes 18a, 18b positioned on the pixel circuits and the dielectric layer 16, a photo conductive layer 20 on the pixel electrodes 18a, 18b, and a transparent electrode 28 on the photo conductive layer 20, wherein the photo conductive layer 20 comprises an n-type layer (n-layer) 22, an intrinsic layer (i-layer) 24, and a p-type layer (p-layer) 26 from bottom to top, which constitute so-called stacked p-i-n layer structure for accepting light and converging light into corresponding charges according to the light intension.
However, the different pixel electrodes 18a, 18b of the prior-art image sensor 10 may have various voltages under illumination, resulted in an electric filed with potential difference between the adjacent pixels 14a, 14b. For example, if the pixel electrode 18b has a high potential VH, and the pixel electrode 18a has a low potential VL under illumination, as the transparent conductive layer 28 is grounded, dark leakage current will occur between the adjacent pixels 14a, 14b, flowing from the pixel electrode 18b with the high potential VH to the nearby pixel electrode 18a with the low potential VL, as shown in FIG. 2. There, the cross-talk problem occurs and influence the accuracy of sensing images, resulted in poor sensing fidelity.
As a result, to improve the structure of the POAP image sensor for avoiding cross-talk problems of adjacent pixels so as to provide a good performance of sensing images is still an important issue for the manufacturers.